The present invention relates to a progressive image signal transmitting apparatus, a progressive image signal receiving apparatus, and a method and a medium each using a bitstream obtained by encoding, e.g., a progressive video signal (sequential scanning signal) based on the MPEG2 formats.
In recent years, the international standardization of MPEG2 (Moving Picture Experts Group the 17-th phase 2) (registration in ISO/IEC 13818-1, 2, 3) has increased the importance of a digital transmission/reception system and method using digital high-efficiency encoding. The use of the MPEG2 encoding scheme has raised expectations for channel multiplexing, improved user interface, and diversified service based on the prospect of additional data transmission, while enabling simultaneous transmission of signals with various image qualities (resolutions) from a single transmitter. As a result, a sequential scanning signal with high image quality (progressive signal) can be transmitted in digital broadcasting.
Below, a description will be given to a conventional progressive signal transmission/reception system.
FIG. 13 is a block diagram showing the structure of the conventional progressive signal transmitting/receiving apparatus.
It is to be noted that, in the following description, an image structure is expressed by using signs i (interlace) and p (progressive) as abbreviations in order to distinguish whether the image structure is interlace or progressive and express a frame rate in a comprehensible manner. By way of example, a progressive signal at a rate of 24 frames per second may be described as 24p and an interlace signal at a rate of 60 frames per second may be described as 60i. 
In FIG. 15, a reference numeral 1301 denotes a view diagrammatically showing the structure of a video material recorded at a sampling rate of 24 frames per second, which is represented by a cinematic film material or the like. A reference numeral 1302 denotes a 24p/60i converter for converting a picture at the rate of 24 frames/second to a normal NTSC television signal at a rate of 60 fields/second. A reference numeral 1303 denotes an MPEG2 video encoder for encoding the aforesaid signal at the rate of 60 fields/second to a bitstream in accordance with the description in ISO 13818-2 (MPEG2-Video). A reference numeral 1304 denotes an MPEG2 video decoder for receiving the aforesaid bitstream and reversely converting the received bitstream to the 60i signal at the rate of 60 fields/second. A reference numeral 1305 denotes a double rate converter for doubling the horizontal scanning rate of the 60i signal.
A description will be given to the operation of the conventional progressive signal transmission/reception system thus structured. Since the film material with the rate of 24 frames/second with no alteration normally requires a special-purpose reproducing device or recording device, it is initially converted to the 60i signal at the rate of 60 fields/second, which is mostly stored in a D1 or S-VHS tape afterward and-distributed.
The 24p/60i converter 1302 operates to convert the signal at the rate of 24 frames/second to a signal at the rate of 60 fields/second by appropriately inserting field repeats in the signal at the rate of 24 frames/second.
FIG. 16 is a view for illustrating the operation of the 24/60i converter. In FIG. 16, it is assumed that a sign A1 represents a field image consisting of the odd-numbered lines (lines 1, 3, 5, . . . ) of a frame A sampled at a time 0 and a sign A2 represents a field image consisting of the even-numbered lines (lines 2, 4, 6, . . . ) of the same frame A.
It is also assumed that a sign B1 represents a field image consisting of the odd-numbered lines of a frame B sampled at a time 1 and a sign B2 represents a field image consisting of the even-numbered lines of the frame B.
The difference between the sampling times 0 and 1 is {fraction (1/24)} seconds in the case of the film material.
Likewise, a sign C1 represents a field image consisting of the odd-numbered lines of a frame C at a time 2 and a sign C2 represents a field image consisting of the even-numbered lines of the frame C.
On the other hand, a sign D1 represents a field image consisting of the odd-numbered lines of a frame D at a time 3 and a sign D2 represents a field image consisting of the even-numbered lines of the frame D.
An example of an output from the 24p/60i converter 1302 is such an output signal as shown in FIG. 16. Specifically, the 24p/60i converter 1302 operates to obtain output images (A1, A2), (A1, B2), (B1, C2), (C1, C2), and (D1, D2) having a 5-frame field structure from input images A, B, C, D having a 4-frame frame structure by inserting a field repeat in each of the frames A and C. A sign (f1, f2) is assumed to indicate that a pair of fields f1 and f2 constitute one frame. A sign f1 represents an image consisting of the odd-numbered lines of the frame, while a sign f2 represents an image consisting of the even-numbered lines thereof. The output signal forms a signal at the rate of 60 fields/second.
The signal at the rate of 60 fields/second is inputted to the MPEG2 video encoder 1303.
The MPEG2 video encoder 1303 operates to convert the. inputted video signal to a bitstream compliant with the format described in ISO/IEC 13818-2 and output the bitstream. The bitstream output is connected to the MPEG2 video decoder 1304 via a transmission system.
At this time, there are cases where the MPEG2 video encoder 1303 encodes the entire frames (30 frames per second) of the inputted signal 60i and outputs the encoded frames without any alterations and where the MPEG2 video encoder 1303 detects a field repeat at the encoder side, performs internal processing so as not to transmit the repeated field, and transmits only data at the rate of 24 frames per second.
In the case where the MPEG video encoder 1303 encodes all the frames of the inputted signal 60i and outputs the encoded frames, the MPEG2 decoder 1303 encodes each frame of the video signal inputted thereto into a bitstream and outputs the bitstream.
In the case where the MPEG2 video encoder 1303 detects the field repeat at the encoder side, performs processing so as not to transmit the repeated field, and transmits only data at the rate of n 24 frames per second, the MPEG2 video encoder 1303 transmits only signals representing the 4 frames of A, B, C, and D but converts each of the signals representing the frames A and C to a bitstream in a form obtained by adding 1 to the repeat_first_field flag bit in accordance with the format described in ISO/IEC 13818-2 and outputs the bitstream.
The MPEG2 video decoder 1304 receives the bitstream produced in accordance with the foregoing procedure and performs a reverse operation in accordance with the grammar described in MPEG2 to reconstruct the video signal from the bitstream.
In the case where the MPEG2 video encoder 1303 detects the field repeat in the input signal, performs processing not to transmit the repeated field, and transmits only data at the rate of 24 frames per second, the MPEG2 video decoder 1304 operates as follows.
In the case where the repeat_first_field flag is 1 in the input bitstream, one field of the target frame is repeatedly outputted. In the case where the repeat_first_field flag is 0, the image signal is outputted without performing a field repeat.
Consequently, the output of the MPEG2 video decoder 1304 is the same signal at the rate of 60 fields/second in either of the cases where the entire frames of the input signal 60i are encoded and outputted and the case where the field repeat is detected at the encoder side, processing is performed not to transmit the repeated field, and only data at the rate of 24 frames per second is outputted, resulting in no difference in terms of frame rate and internally constructed image.
The rate doubling unit 1305 receives the output of the MPEG2 video decoder and transforms the signal at the rate of 60 fields/second to a signal having a scanning rate double the scanning rate for a line scanning line, i.e., a frame rate of 60 frames/second.
FIGS. 17(a) and 17(b) are for illustrating inputs and outputs of the rate doubling unit 1305. In FIGS. 17(a) and 17(b), it is assumed that the signs A1, A2, B1, B2, C1, C2, D1, D2, A, B, C, and D represent the same images as represented by the signs in FIG. 16. As a technique for rate doubling, there is one implemented by a repeat in a field structure and one implemented by a repeat in a frame structure. FIG. 17(a) is for illustrating the case where rate doubling is implemented by a repeat in a field structure. FIG. 17(b) is for illustrating the case where rate doubling is implemented by a repeat in a frame structure.
In the case where rate doubling is implemented by a repeat in a field structure, the field A1 which is intrinsically image data for an odd field is also used for an even field, thereby achieving effect that a frame at the sampling time for the field A1 is composed of a single field, as shown in FIG. 17(a). This enables the obtention of a double rate signal at the rate of 60 frames/second from a signal at the rate of 60 fields/second.
In the case where rate doubling is implemented by a repeat in a frame structure, the field A1 and the field A2 immediately subsequent thereto combine to compose one frame, thereby achieving the effect that the same frame as outputted at the sampling time for the field A1 is also outputted at the sampling time for the field A2. This enables the obtention of a double rate signal at the rate of 60 frames/second from a signal at the rate of 60 fields/second.
In the case shown in FIG. 17(a), however, the foregoing conventional structure presents the problem that, though the temporal order of display is Axe2x86x92Bxe2x86x92Cxe2x86x92D and correct, image quality in the vertical direction is significantly degraded because the same field data is written twice in each of the frame structure.
In the case shown in FIG. 17(b), the conventional structure also presents the problem that image quality is degraded because a frame composed of a combination of fields not sampled at the same time, such as (A1, B2) or (B1, C2), is sporadically present and an inter-field difference is particularly large in a scene involving a dynamic motion in the direction of a time axis, though image quality is not degraded in the vertical direction.
In view of the foregoing conventional problems, it is therefore an object of the present invention to provide a progressive image signal transmitting apparatus, a progressive image signal receiving apparatus, and a method and a medium each capable of obtaining, even when an inter-field motion is dynamic, a double rate signal superior to that obtained conventionally and maintaining vertical resolution.
To attain the object, one aspect of the present invention is a progressive image signal receiving apparatus comprising: video decoding means for receiving a bitstream, converting the bitstream to an interlaced video signal and outputting the video signal; frame structure analyzing means for calculating, based on respective signals representing a current frame and a frame which is 1 frame time previous to the current frame each outputted from the video decoding means, a difference between the both frame signals and outputting a specified control signal based on a result of the calculation; and rate doubling means for producing a frame signal based on the video signal outputted from the video decoding means and on the control signal outputted from the frame structure analyzing means to provide a progressive signal at a scanning rate which is a specified number of times as high as a scanning rate of the output signal from the video decoding means.
Another aspect of the present invention is a progressive image signal receiving apparatus comprising: video decoding means for receiving a bitstream and outputting an interlaced video signal; temporary storing means for receiving the output signal from the video decoding means and delaying the signal by a 1 frame time; judging means for calculating, based on a current signal outputted from the video decoding means and a previous frame signal outputted from the temporary storing means, a difference between the both frame signals and outputting a specified control signal based on a result of the calculation; and rate doubling means for producing a frame signal based on the video signal outputted from the temporary storing means and on the control signal outputted from the judging means to provide a progressive signal at a scanning rate which is a specified number of times as high as a scanning rate of the output signal from the temporary storing means.
Still another aspect of the present invention (corresponding to the invention defined in claim 5) is a progressive image signal receiving apparatus, wherein the frame structure analyzing means calculates, in calculating the difference between the both frame signals, the sum of the absolute values of differences between pixels in a part of the current frame and pixels in a part of the previous frame.
Yet another aspect of the present invention is a progressive image signal receiving apparatus comprising: video decoding means for receiving a bitstream, converting the bitstream to a video signal of progressive type, and outputting the video signal; and flag bit analyzing means for detecting, in the bitstream, a flag signal indicating the presence or absence of a field repeat described correspondingly to a frame signal and outputting, to the video decoding means, the presence or absence of the field repeat indicated by the detected flag signal, if the output from the flag bit analyzing means indicates the presence of the field repeat, the video decoding means repeatedly outputting the same frame signal as the frame signal corresponding to the flag signal and outputting the video signal of progressive type at a scanning rate which is a specified number of times as high as a scanning rate of the frame signal in the bitstream.
Still yet another aspect of the present invention (corresponding to the invention defined in claim 7) is a progressive image signal transmitting apparatus for transmitting a video material shot by a sequential scanning (progressive scanning) method in accordance with a bitstream defined in ISO/IEC 13818-2, the transmitting apparatus describing a frame rate of the video material by using a bit position in a user data region in the bitstream and transmitting the frame rate.
A further aspect of the present invention is a progressive image signal receiving apparatus for receiving a bitstream transmitted from a transmitting apparatus and outputting a sequential scanning signal to a progressive monitor, the receiving apparatus comprising: second flag bit analyzing means for recognizing a frame rate of a video material from bit data at a given bit position predefined by agreement with the transmitting apparatus; and video decoding means for receiving an output of the second flag bit analyzing means and the bitstream, determining the frequency of frame repeats based on a ratio between the output of the second flag bit analyzing means and a frame rate displayed on the progressive monitor, reconstructing an image, and outputting a progressive signal.
A still further aspect of the present invention is a progressive image signal receiving apparatus for receiving a bitstream defined in ISO/IEC 13818-2 and outputting a progressive signal, the receiving apparatus comprising: flag bit analyzing means for analyzing whether or not a code value described in a frame_rate_code flag in a sequence_header is half the value of a frame rate of the progressive signal outputted and outputting a result of the analysis as a control signal; video decoding means for receiving the bitstream and reconstructing an image based on the definition in ISO/IEC 13818-2; and rate doubling means for converting, based on the control signal outputted from the flag bit analyzing means, an output signal from the video decoding means to a signal at a scanning rate which is double the scanning rate of the output signal.
With such structures, it is judged whether or not a decoded frame is a frame to be repeatedly displayed at the receiving apparatus and control operation is performed based on the judgment. This achieves double rate display without causing the degradation of image quality.